interface llist {} class Nil : llist {} class Cons : llist { public A head; public llist tail; public Cons(A h, llist t) {head=h; tail=t;} } public class llists { static string check(llist m) { string answer = ""; if (m is Nil) answer = "empty"; else if (m is Cons) { if (((Cons)m).tail is Cons) answer= "at least two"; else answer = "just one"; } return answer; }//checkp public static void Main() { System.Console.WriteLine( check(new Cons(2,new Nil())) ); } }